The present invention relates to a ring laser gyroscope. More particularly, it relates to an arrangement of piezoelectric transducers for driving a dither spring, and for sensing the flexure of the dither spring commonly used in dither motors for ring laser gyroscopes.
Piezoelectric dither motors are commonly used for the purpose of dithering laser gyro blocks. Typically, a dither motor is comprised of one or more piezoelectric transducers (PZTs) attached to a dither spring as shown and described in U.S. Pat. No. 4,370,583 issued to Ljung. The dither spring is generally composed of a central member or hub having a plurality of flexible radial members or spokes extending between the central member and a toroidal rim.
In the aforementioned patent, a single piezoelectric element is comprised of upper and lower surfaces. One surface electrode is provided on the lower surface of the piezoelectric element. Two other surface electrodes are attached to the piezoelectric element at opposite ends of the upper surface of the piezoelectric element. The two upper electrodes are separated by a gap which is at the midpoint of the piezoelectric element. The piezoelectric element with the two electrodes is rigidly attached to a spoke such that the gap between the electrodes is in juxtaposition with the the point of inflection of the radial member. The lower electrode surface and the adjacent lower surface of the piezoelectric element is attached to a spoke by an insulating epoxy. The piezoelectric element is positioned on the spoke such that its midpoint corresponds to the point of inflection or a point of neutral radius of the spoke. At the point of inflection, when the spoke is flexed, the spoke is negligibly deviated from its rest position.
With the piezoelectric element of Ljung described above, a voltage is applied across the upper surface electrodes causing the PZT to deform and flex the spoke. By application of a generally sinusoidal voltage, a push-pull effect of the PZT on the spoke is achieved. This combined push-pull/flexing effect oscillates the rim relative to the hub. Generally, the dither spring is mounted to a laser gyro block to cause rotational oscillations of the block relative to an inertial platform.
Further, as shown and described in U.S. Pat. No. 4,406,965 issued to Ljung, a separate and non-driven pickoff PZT, constructed in a manner as already described with reference to U.S. Pat. No. 4,370,583, may be attached to one of the spokes for sensing dither rotation. In these circumstances, the two surface electrodes mounted on the upper surface of the piezoelectric element are electrically connected to a detection circuit to provide an output signal related to the amount of flexure of the spoke. The amount of flexure of the spoke is of course indicative of the magnitude of the rotation of the rim relative to the central member.
The output of the detection circuit is sometimes referred to as the "dither pick-off signal" As is well known in the art, the pickoff signal may be used as part of a closed-loop control circuit for oscillating the spring-mass system of the gyroblock and the dither motor at resonance. Also, the dither pickoff is commonly used for "dither signal stripping" schemes which remove the dither rotation component from the usual readout rotation signal.
In the aforementioned prior art dithering scheme, the arrangement of the PZT electrodes can lead to undesired crosstalk which degrades the performance of the dither motor and also contributes to errors in the pickoff signal. Further, as gyro size decreases, dither motor size typically decreases. Hence, in smaller dither motors, the area of the gap between electrodes correspondingly decreases which increases the possibility of crosstalk.
Further, in the prior art, the employment of a pickoff PZT on a spoke, particularly as described in the above referred to patents, precluded the employment of a driving PZT on the same spoke as the sensing PZT. This is particularly illustrated and described in the aforementioned Ljung patents.